This invention relates to a method and apparatus for the underground mining of bituminous sands, oil shales and other friable mineral deposits.
The recovery of petroleum from naturally occurring bodies of bituminous sands and shales has long presented problems of practical significance because of the mining problem involved. In for example the Athabasca area of Alberta, Canada, there occur extensive deposits of sediments, known as the "McMurray sediments", which are deposited on Pre-Cretaceous erosion surfaces generally of Devonian limestone. The sediments comprise relatively coarse sands, overlain by a mantle of glacial drift, which varies in depth from a few feet adjacent the Athabasca river, to in excess of 1,800 feet at a distance of several miles from the Athabasca river.
The McMurray sediments extend over approximately 9,000 square miles, in only approximately 7% of which is the overburden less than 100 feet in depth, and in only 20% of this area is it less than 250 feet (Alberta Oil & Gas Conservation Board "A Description and Reserve Estimate of the Oil Sands of Alberta, 1963").
At some time during the geological history of the McMurray sediments, they were invaded by some 600 billion barrels of oil. The McMurray sediments themselves extend from 100 to 200 feet in thickness, and the oil content of the sediments is roughly one barrel of oil per cubic yard of sands; one cubic yard of oil-saturated sand weighs approximately one ton.
The top of the McMurray formation is relatively flat, and its varying thickness is due to the topography of the Pre-Cretaceous erosional surface on which it rests.
Heretofore, two separate and distinct approaches have been used, or conceived in exploiting the McMurray oil sands. Initially, and to date the only commercial method has been that of surface mining of the sands, in which a location is selected where the overburden does not extend to a depth greater than approximately 20 feet. The overburden is stripped with draglines or scrapers, to expose the top of the oil sands, and various mining techniques are then employed in order to mine the sands, which are then transported by conveyor or mine trucks to a separation plant, for separating the oil from the coarse sands. Large shovels, mining wheels, rippers and scrapers have heretofore been employed in the mining process. The Great Canadian Oil Sands Ltd. plant at Mildred Lake, Alberta, utilizes large mining wheels, mounted in outboard fashion on booms, in which cutter-equipped buckets are rotated around a wheel, the cutters engaging the exposed oil sands face on a mining bench, each cutter-equipped bucket slicing a cut of several inches of sand from the face.
The economics of such surface mining procedures have restricted the operations to areas of relatively low overburden, and it has generally been conceived that development of the major portion of the oil sands must depend on in situ methods. Such in situ methods have been experimental, and generally have involved underground combustion or solution mining, in which a diluent such as kerosene, is injected into the formation through input boreholes for recovery of the diluent and entrained oil from adjacent output boreholes. Although the economic results of such in situ methods have not been published, it is generally believed that such methods are not commercially practicable because of the relatively high losses of the recovery agents employed.
The heavy asphaltic oil in the oil sands is the bonding agent which consolidates the sand into a quasi-sandstone. The oil is heavy gravity (from 8.degree. API to 12.degree. API), viscous especially at low temperatures; the formation temperature of the oil sands is around 40.degree.F.
Although the oil has much the same general properties throughout the formation, there are some differences. In the McMurray area, the oil has specific gravities of 1.020 to 1.025 and is much more viscous than the oil from elsewhere in the formation, where the oil has specific gravities of 1.005 to 1.010. Oil of specific gravities of 1.000 or somewhat less have been observed at the bottom of the formation.
The only commercial plant to date in the McMurray area, that of Great Canadian Oil Sands Ltd., has a design capacity of 100,000 tons per day of mined sand. Engineering estimates of optimum plant sizes suggest that only large scale operation of at least 100,000 tons of sand per day can be expected to reduce unit costs to the point where the recovered bitumen will be competitive with conventionally obtained crude oil.
Another huge source of synthetic crude oil occurs in the Colorado oil shales in which the hydrocarbon occurs in shale beds, the costs of production appearing to significantly exceed the costs of oil production from the Athabasca oil sands.
It is basic to the concept of this invention, that, except for the relatively small areas adjacent the Athabasca river of low overburden, and therefore amenable to surface mining, any economic recovery of the major portions of the McMurray bitumen body will depend on the development of an economic underground mining method.